This invention relates, in general, to methods for automatically inspecting semiconductor packages, and more particularly, to a method for automatically inspecting high lead count semiconductor packages.
During the assembly of semiconductor components it is necessary to verify the location, alignment, and condition of various portions of the package. This is particularly true in an automated or semi-automated assembly cycle. In the manufacture of semiconductor devices it is also important to inspect the semiconductor package before shipping to ensure that package lead-fingers are in proper alignment and have not been damaged during assembly. Recently, solid state television cameras have been used to visually monitor semiconductor packages. The data from the camera can then be processed in a digital manner to determine the coordinate location of the semiconductor package and leads. However, the manner in which the data is processed determines speed, accuracy, and robustness of the monitoring system.
A digital image from an industrial camera is broken into picture elements, or pixels. Even when gray scale images are used, each pixel requires a byte of digital data to describe its intensity; commonly an unprocessed digital image will require in the order of 256 thousand bytes of data. Digital processing of a visual image for automatic inspection involves reducing this large amount of image data to a minimum data set while retaining information about important characteristics of the package. Important characteristics are lead-finger dimensions and spacings which must meet predetermined specifications. To determine acceptability of the semiconductor package under inspection the data set must be compared to some standard or specification. One common problem in image analysis is that results are very sensitive to background lighting, reflectivity, and magnification of the image under inspection. Since these conditions are quite variable in a production environment, image comparison or image correlation techniques are difficult to use in a production inspection operation and require a great deal of manual intervention. It is desirable to have an automatic inspection system which is highly immune to the effects of variable lighting conditions, and which is highly tolerant of normal, acceptable variations between leadframes while being quite sensitive to unacceptable variations.
In particular, high lead count semiconductor packages pose difficult problems in both data reduction and data analysis. Due to the large number of leads which must be inspected, data reduction to a minimal data set is necessary for speedy processing and analysis. On the other hand, this data reduction must not impair the systems ability to detect unacceptable defects in the package. Characteristics of primary importance are lead-finger count, lead alignment and lead spacing.
Accordingly, it is an object of the present invention to provide an improved method of manipulating data to automatically inspect a high lead count semiconductor package.
Another object of the present invention is to provide a method to automatically inspect a large number of lead-fingers in a manner that is fast and accurate.
Yet another object of the present invention is provide a method to analyze visual data against a predetermined specification in a real time fashion.
Another object of the present invention is to use image enhancement techniques together with logical AND operations to vastly reduce size of a data set which describes characteristics of a leadframe.
Still another object of the present invention is to use morphological dilation and skeletonization techniques to increase the robustness of an image analysis system.